Various devices and methods are provided for assisting a driver by detecting surroundings of a vehicle in order to improve safety and operability of the vehicle. In order to assist the driver in the above-described way, various sensors such as a grayscale image sensor, a range image sensor and the like, which detect a visual image and three-dimensional information around the vehicle, are provided at the vehicle. Additionally, above-mentioned sensors, which function as human eyes, are used for robots. In a case where the above-mentioned sensors are provided to a mobile body such as the vehicle, the robots and the like, in order to recognize three-dimensional characteristics of the surroundings while the mobile body is moving, a sensor position and a sensor posture relative to a coordinate system, in which the mobile body is set as a reference, may preferably be accurately calibrated. The range image sensor, which detects the three-dimensional circumstances, is often used as a sensor for detecting surroundings of the mobile body. Therefore, calibration of high accuracy is required for such range image sensors.
A method of calibrating a three-dimensional measuring device, is as an example of the range image sensor, is disclosed in JP10-47920A. According to the method of calibrating the three-dimensional measuring device disclosed in JP 10-47920A, a measurement plate, which serves as a subject of calibration, is moved relative to the three-dimensional measuring device. More specifically, the measurement plate is moved so as to rotate in a yaw direction, so as to rotate in a pitching direction, and so as to move forward and backward. The three-dimensional measuring device obtains a range image at each moving point. Then, a three-dimensional coordinate value is calculated on the basis of a range image value of each pixel (picture element), so that a calibration value is obtained from the three-dimensional coordinate value.
However, in the method of calibrating the three-dimensional measuring device disclosed in JP 10-47920A, the measurement plate needs to be moved. Accordingly, accuracy for moving the measurement plate may affect calibration accuracy. In order to improve the accuracy for moving the measurement plate, a moving means for the measurement plate needs to be carefully controlled, which may result in increasing a calibration system in size. Further, time necessary for calibration also becomes longer. In a case where an automobile is adapted as the moving body, it is not preferable to extend time for a calibration process in a production line in perspective of production efficiency. Further, the method disclosed in JP 10-47920A is a method of calibrating the single three-dimensional measuring device, and is not a method of calibrating the three-dimensional measuring device in a state where the three-dimensional measuring device is mounted on the mobile body. Even if the three-dimensional measuring device (the range image sensor) itself is calibrated with high accuracy, the coordinate system of the range image sensor may be displaced relative to the reference coordinate system of the mobile body when the three-dimensional measuring device is provided to the mobile body. Hence, the calibration of the range image sensor may preferably be executed in the state where the range image sensor is mounted on the mobile body.
A need thus exists to provide a calibration device and a calibration method for a range image sensor, which are not susceptible to the drawback mentioned above.